Multiple input multiple output (MIMO) technology is one of important physical layer technologies for a long-term evolution/long-term evolution advanced (LTE/LTE-A) system, which is used for providing spatial diversity gains, spatial multiplexing gains and array gains.
When a base station and a mobile station are configured with multiple antennas, a multi-antenna transmission scheme may be properly selected according to such situations as traffic demands, channel states and particular antenna configurations, so as to improve the performance of the system. In the study of LTE-A downlink MIMO enhancement technology, three scenarios are defined, which are a macro cell scenario, indoor and/or outdoor low-power node scenario, and a small cell scenario of outdoor centralized configured antennas, respectively.
In the first scenario, i.e. the macro cell scenario, a base station end supports three types of antenna configurations, 2 antennas, 4 antennas, and 8 antennas, and the antenna configurations with 2 antennas and 4 antennas has a higher priority than the antenna configuration with 8 antennas. In the second and third scenarios, low-power nodes may be configured with 1 antenna (which is inapplicable to the third scenario), 2 antennas and 4 antennas, and the antenna configuration with 4 antennas has a higher priority than the antenna configurations with 1 antenna and 2 antennas. It can be seen therefrom that the antenna configuration with 4 antennas is important configuration in the study of LTE-A multi-antenna technology. In the above three scenarios, the 4 transmission antennas at the transmission end support four types of particular configuration, as shown in FIGS. 1A to 1D.
FIGS. 1A to 1D are schematic diagrams of antenna configurations in an antenna system. Wherein, FIG. 1A shows cross-polarized antennas with an antenna spacing of 0.5 times wavelength (λ), FIG. 1B shows cross-polarized antennas with an antenna spacing of 4λ, FIG. 1C shows co-polarized antennas with an antenna spacing of 0.5λ, and FIG. 1D shows co-polarized antennas with an antenna spacing of 4λ. And wherein, FIGS. 1C and 1D show horizontally co-polarized antenna configurations in co-polarized antenna configuration, and furthermore, the co-polarized antenna configuration may further include configurations with antennas that are vertically placed or placed at angles of ±45°.
In the study of LTE-A Release 12, the feedback enhancement technology based on codebooks in 4-antenna configuration is one of the focuses. The object of the study is to realize 4-antenna precoding matrix index (PMI) feedback codebook enhancement, so as to provide finer spatial domain granularities and to support different antenna configurations in macro cell and small cell scenarios, especially small and large spacing cross-polarized antennas, and co-polarized distributed antenna configuration of imbalanced power.
Due to different antenna configurations, needed optimal precoding matrices are also different. Wherein, precoding codewords applicable to co-polarized antennas (i.e., uniform linear array, ULA) with a spacing of 0.5λ have the following structures respectively:
                                                        v              ULA                        ⁡                          (              θ              )                                =                      [                                                            1                                                                                                  ⅇ                    jθ                                                                                                                    ⅇ                    j2θ                                                                                                                    ⅇ                    j3θ                                                                        ]                          ;                            (        1        )            
precoding codewords applicable to cross-polarized antennas (XPOL) with a spacing of 0.5λ have the following structures respectively:
                                                        v              XPOL                        ⁡                          (                              θ                ,                α                            )                                =                      [                                                            1                                                                                                  ⅇ                    jθ                                                                                                α                                                                                                  α                    ⁢                                                                                  ⁢                                          ⅇ                      jθ                                                                                            ]                          ;                            (        2        )            
and precoding codewords applicable to antennas with a spacing of 4λ (including co-polarization and cross polarization) have the following structures respectively:
                                          v            UNCORR                    ⁡                      (                                          θ                1                            ,                              θ                2                            ,              α                        )                          =                              [                                                            1                                                                                                  ⅇ                                          jθ                      1                                                                                                                    α                                                                                                  α                    ⁢                                                                                  ⁢                                          ⅇ                                              jθ                        2                                                                                                                  ]                    .                                    (        3        )            
In the structure in above (1) to (3), θ denotes a phase, θ1, θ2 being uniformly quantized between [0, 2π), and αε{1, −1, j, −j}, denoting a phase weighting factor.
In the protocols of Release 10 and the earlier, 4-antenna MIMO employs a single codebook structure, as shown in FIG. 1. Wherein, ui(i=0, 1, . . . , 15) are base vectors, and Wi is a matrix generated by performing Householder transformation on ui all precoding matrices (i.e. codewords) with ranks 1-4 being able to be obtained from related columns of Wi. When the rank of the system is 1, codewords numbered 0-7 may be expressed as
            [                                    1                                              ⅇ                                                j2π                  ·                  1                  ·                  m                                8                                                                        ⅇ                                                j2π                  ·                  2                  ·                  m                                8                                                                        ⅇ                                                j2π                  ·                  3                  ·                  m                                8                                                        ]        T    ,values of m being 0, 2, 4, 6, 1, 3, 5, 7, respectively, that is, the former 8 codewords may be applicable to a scenario of co-polarized antennas with a spacing of 0.5λ, and codewords numbered 0-11 may be expressed as
            [                                    1                                              ⅇ                                                j2π                  ⁢                                                                          ⁢                  m                                8                                                          α                                              αⅇ                                                j2π                  ⁢                                                                          ⁢                  m                                8                                                        ]        T    ,that is, they are applicable to a scenario of cross-polarized antennas with a spacing of 0.5λ, where,
      θ    =                  2        ⁢        π        ⁢                                  ⁢        m            8        ;αε{1, −1, j, −j}. Furthermore, as a minimum chordal distance between 16 codewords in a codebook is 0.707, better performance may be obtained in uncorrelated channels. In summary, the 4-antenna codebook in Release 10 is applicable to all the four antenna configurations shown in FIGS. 1A-1D. However, as the number of the codewords is relatively small (which is only 16), division of spatial domain granularity in various antenna configurations is insufficiently fine.
TABLE 14-antenna Codebook in Release 10 Codebook for transmission on antenna ports {0,1, 2, 3} and for CSI reporting based on antenna ports {0, 1, 2, 3} or {15, 16, 17, 18}.CodebookNumber of layers υindexun12340u0 = [1 −1 −1 −1]TW0{1}W0{14}/{square root over (2)}W0{124}/{square root over (3)}W0{1234}/21u1 = [1 −j 1 j]TW1{1}W1{12}/{square root over (2)}W1{123}/{square root over (3)}W1{1234}/22u2 = [1 1 −1 1]TW2{1}W2{12}/{square root over (2)}W2{123}/{square root over (3)}W2{3214}/23u3 = [1 j 1 −j]TW3{1}W3{12}/{square root over (2)}W3{123}/{square root over (3)}W3{3214}/24u4 = [1 (−1 − j)/{square root over (2)} −j (1 − j)/{square root over (2)}]TW4{1}W4{14}/{square root over (2)}W4{124}/{square root over (3)}W4{1234}/25u5 = [1 (1 − j)/{square root over (2)} j (−1 − j)/{square root over (2)}]TW5{1}W5{14}/{square root over (2)}W5{124}/{square root over (3)}W5{1234}/26u6 = [1 (1 + j)/{square root over (2)} −j (−1 + j)/{square root over (2)}]TW6{1}W6{13}/{square root over (2)}W6{134}/{square root over (3)}W6{1324}/27u7 = [1 (−1 + j)/{square root over (2)} j (1 + j)/{square root over (2)}]TW7{1}W7{13}/{square root over (2)}W7{134}/{square root over (3)}W7{1324}/28u8 = [1 −1 1 1]TW8{1}W8{12}/{square root over (2)}W8{124}/{square root over (3)}W8{1234}/29u9 = [1 −j −1 −j]TW9{1}W9{14}/{square root over (2)}W9{134}/{square root over (3)}W9{1234}/210u10 = [1 1 1 −1]TW10{1}W10{13}/{square root over (2)}W10{123}/{square root over (3)}W10{1324}/211u11 = [1 j −1 j]TW11{1}W11{13}/{square root over (2)}W11{134}/{square root over (3)}W11{1324}/212u12 = [1 −1 −1 1]TW12{1}W12{12}/{square root over (2)}W12{123}/{square root over (3)}W12{1234}/213u13 = [1 −1 1 −1]TW13{1}W13{13}/{square root over (2)}W13{123}/{square root over (3)}W13{1324}/214u14 = [1 1 −1 −1]TW14{1}W14{13}/{square root over (2)}W14{123}/{square root over (3)}W14{3214}/215u15 = [1 1 1 1]TW15{1}W15{12}/{square root over (2)}W15{123}/{square root over (3)}W15{1234}/2
A dual codebook is a relatively new type of precoding structure over the single codebook structure employed by the 4-antenna systems in Release 10. There exist two codebooks in the dual codebook system, one is used for long-term and/or wideband feedback, and the other is used for short-term and/or instantaneous subband feedback. Complex codewords of the system are obtained from a product of codewords in the two codebooks, and the 8-antenna codebook design in Release 10 employs such the dual codebook structure. In the 4-antenna codebook design in current Release 12, there mainly exist two types of dual codebooks, a GoB (grid of beam) codebook and a differential codebook.
The GoB codebook is designed on a basis of an idea of beams, which is applicable to small spacing cross-polarized antenna configuration. W=W1W2, W1 is a block diagonal matrix, each sub-matrix containing multiple adjacent overlapped beams and W1 used for long-term and wideband feedback, and W2 contains beam selection vectors and phase weighting factors; wherein a unique beam is selected from the multiple beams contained in W1, and phase weighting is performed on two polarized antenna groups, and W2 is used for short-term and/or narrowband feedback. Different from the GoB codebook, a W=W2W1 structure is used in the differential codebook, W1 is a base matrix (vector) used for long-term and/or wideband feedback, for example, the 4-antenna codebook in Release 10 (Table 1) may be used; and W2 is a unitary diagonal matrix used for tracing fluctuation of W1, and is instantaneous and/or subband feedback.
However, the existing dual codebook scheme is inapplicable to various antenna configuration scenarios.
It should be noted that the above description of the background is merely provided for clear and complete explanation of the present disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of the present disclosure.